There is increasing interest in the empirical effects of prayer on health. However, little attention has been paid to the theoretical causal mechanisms by which health may be affected by prayer. A critical examination of extant literature on the psychology of prayer was conducted to identify potential ways in which prayer may act to promote health. The review suggested that an array of prayer types exist ranging from prayers of attunement to petitionary prayers. Additionally, prayer may affect health by a variety of means including: (a) prayer may improve health because of the placebo effect; (b) individuals who pray may also engage in health-related behaviour; (c) prayer may help by diverting attention from health problems; (d) prayer may promote health through supernatural intervention by God; (e) prayer may activate latent energies, such as chi, which have not been empirically verified, but which nevertheless may be beneficial to health; and (f) prayer may result in a unity of consciousness which facilitates the transmission of healing between individuals. An agreement among researchers about the classification and measurement of prayer might result in an easier comparison of results across studies. Researchers should be aware of the many prayer types documented in the literature and should select a measure of prayer appropriate to their field of investigation. Additionally, researchers should give consideration to the possible causal mechanisms underlying the hypotheses they are investigating. Furthermore, if researchers offer some theoretically based argument, regarding prayer, for the hypotheses that they are testing, then results from studies involving prayer may be more meaningful. Having established the theoretical models of the ways in which prayer may promote better health, the challenge for psychology is to provide empirical evidence to confirm or disconfirm the alternate hypotheses.